Mathematics – Algebraic Geometry
Scientific paper
2010-01-09
Mathematics
Algebraic Geometry
27 pages, new version, cosmetic changes, a new geometric proof of the description of the normalized moment map is added (Secti
Scientific paper
Let $G$ be a connected reductive group acting on an irreducible normal algebraic variety $X$. We give a slightly improved version of local structure theorems obtained by F.Knop and D.A.Timashev that describe an action of some parabolic subgroup of $G$ on an open subset of $X$. We also extend various results of E.B.Vinberg and D.A.Timashev on the set of horospheres in $X$. We construct a family of nongeneric horospheres in $X$ and a variety $\Hor$ parameterizing this family, such that there is a rational $G$-equivariant symplectic covering of cotangent vector bundles $T^*\Hor \dashrightarrow T^*X$. As an application we get a description of the image of the moment map of $T^*X$ obtained by F.Knop by means of geometric methods that do not involve differential operators.
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